This application claims priority from China Patent Application Number 01131953.4 which was filed on Oct. 22, 2001.
1. Field of the Invention
The present invention relates to a process for the selective disproportionation of toluene, and the disproportionation and transalkylation of toluene and C9+ aromatics (C9+A), in particular it relates to a process for the selective disproportionation of toluene, and the disproportionation and transalkylation of toluene and C9+ aromatics followed by a process for producing p-xylene (PX) by a technique of the isomerization of C8 aromatics (C8A) and a technique of p-xylene separation.
2. Description of the Related Art
p-xylene is one of the major basic organic feedstocks in petrochemical industry and has widespread applications in many fields such as chemical fiber, synthetic resin, pesticide, medicine, plastic, etc. The traditional process for producing p-xylene is shown in FIG. 1. C8 aromatics (C8A) in thermodynamic equilibrium produced from the catalytic reforming process of naphtha passes through a multi-stage subzero crystallization separation or molecular sieve simulation moving bed separation (abbreviated as adsorptive separation) to separate p-xylene from its isomer mixture with near boiling points. A C8A isomerization (abbreviated as isomerization) process is generally used to isomerize o-xylene and m-xylene to p-xylene. The use of disproportionation of toluene, or the disproportionation and transalkylation of toluene and C9+ aromatics (C9+A) to produce benzene and C8A, is an effective route for increasing the output of p-xylene.
So far, the typical and mature processes relating to toluene disproportionation in the world are the Tatoray traditional toluene disproportionation process of UOP Inc. industrialized in the end of 1960s, the MTDP process of Mobil Chemical Company developed in the end of 1980s, the S-TDT process of Shanghai Institute of Petrochemical Industry developed recently, and the TransPlus transalkylation process of heavy aromatics of Mobil Chemical Company. The selective disproportionation of toluene is a new route for producing p-xylene. Since the selective disproportionation of toluene on modified ZSM-5 catalysts can produce benzene and C8A with a high concentration of p-xylene, the difficulty in the separation of p-xylene is greatly alleviated. In recent years, along with the improvement of the catalyst performance, this process has made a great progress. The typical processes are the Mobil toluene selective disproportionation process MSTDP industrialized in late 1980s and the UOP toluene selective disproportionation process PX-Plus developed in late 1990s.
In the industrialized toluene selective disproportionation process MSTDP, a converted ZSM-5 mesoporous molecular sieve is used as the catalyst to treat a toluene feedstock yielding C8A with a high concentration of p-xylene (85-90% by weight, the same below except otherwise noted) and nitration grade benzene. In the PX-Plus process, no industrialization of which has been reported, to applicant""s knowledge, the major reaction performance is a PX selectivity of 90%, a mole ratio of benzene to PX of 1.37 in case of the toluene conversion of 30%.
Nevertheless, in these toluene selective disproportionation processes, a high p-xylene selectivity is accompanied by a harsh requirement to the feedstock selection, and only toluene can be used as the feedstock. C9+A has no use for these processes, and at least it can not be directly used. This is a vital shortcoming of the selective disproportionation process.
The feed for the reactor of a typical Tatoray process is toluene and C9 aromatics (C9+A). The xylenes produced in the Tatoray process are a mixture of the isomers in thermodynamic equilibrium, and generally, the content of p-xylene having the most industrial application value is only about 24%. Compared to the selective disproportionation of toluene wherein mixed xylenes having a p-xylene concentration of about 90% can be obtained, the Tatoray process is obviously inferior in this aspect, but a great advantage of the Tatoray process over the toluene selective disproportionation process is that the Tatoray process can convert C9A to benzene and xylenes.
The literature based on the Tatoray Process includes U.S. Pat. Nos. 4,341,914, 2,795,629, 3,551,510, CN98110859.8, CN97106719.8, etc. The process disclosed by the representative U.S. Pat. No. 4,343,914 comprises conducting the aromatics fractionation of the reformed product, feeding the derived toluene and C9A to the Tatoray unit to conduct disproportionation and transalkylation reactions, recycling toluene, C9A, and a part of C10 aromatics (C10A) after separating the products, withdrawing benzene as a product, feeding xylene together with the xylene from the isomerization unit to the PX separation unit to separate the high purity of p-xylene product. The other isomers of xylene are fed to the isomerization unit to conduct the isomerization reaction of xylene, yielding the mixed xylenes in thermodynamic equilibrium again.
One representative aromatics complex based on the selective disproportionation process is the PX-Plus Process. The greatest difference between the PX-Plus Process and the traditional aromatics complex Tatoray process is that C9+A is withdrawn as a byproduct instead of as a feed in the PX-Plus process.
It is readily seen from summarizing the above processes that the traditional aromatics production process uses the disproportionation and transalkylation process to attain the object of increasing the output of xylenes, but the amount of the recycled xylenes is large since the produced mixed xylenes are the isomers of xylene in the thermodynamic equilibrium and therefore the concentration of p-xylene is low. The other isomers of xylene have to pass through the isomerization unit to be converted to p-xylene, thus resulting in heavy xylene recycles and high energy consumption in the isomerization unit, PX separation unit and aromatics fractionation unit. Although mixed xylenes with high concentration of p-xylene can be obtained by the aromatics production process using the selective disproportionation process, and thereby the feeds of the isomerization unit and the like are greatly reduced, the selective disproportionation process can not treat C9+A, which results in a waste of the C9+A resource, and a reduced output of the target product, p-xylene.
Therefore, it is very desirable to develop a process for producing high yield of the desire product p-xylene, which does not have harsh strict requirement to the feedstock. It is also desirable to greatly reduce the energy consumption or increase the capacity of processing the feedstock.
Thus, one object of the present invention is to overcome the shortcomings present in the prior art of low concentration in the mixed xylenes, high energy consumption, or harsh requirement to the reaction feedstock in the production of p-xylene.
Another object of the present invention is to provide a novel process for the selective disproportionation of toluene and the disproportionation and transalkylation of toluene and C9+ aromatics. The improved economical viability and profitability of producing p-xylene are obtained by increasing the capacity of the whole process and the output of the target products p-xylene and benzene, greatly reducing the scale of the p-xylene separation unit, isomerization unit, and the aromatics fractionation unit, and thereby decreasing the energy consumption of the whole process.
These and other objects are attained by a novel process of the selective disproportionation of toluene and the disproportionation and transalkylation of toluene and C9+ aromatics comprising:
a) separating a feed stream comprising benzene, toluene, C8 aromatics, and C9+ aromatics into a first benzene stream, a toluene stream, a first C8 aromatics stream, and a C9+ aromatics stream;
b) introducing a part of the toluene stream into a toluene selective disproportionation unit to conduct the toluene selective disproportionation reaction for producing a first effluent comprising C8 aromatics rich in p-xylene, and benzene;
c) separating a second C8 aromatics stream and a second benzene stream from the first effluent produced in the step of b);
d) introducing another part of the toluene stream and the C9+ aromatics stream into a toluene disproportination and transalkylation unit to conduct the toluene disproportionation and transalkylation reaction in the presence of hydrogen for producing a second effluent comprising C8 aromatics and benzene;
e) separating a third C8 aromatics stream and a third benzene stream from the second effluent produced in the step of d); and
f) separating the first, and third C8 aromatics stream into a p-xylene product and a remaining mixed xylenes stream.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate procedures described herein.